Method for manufacturing flexible circuit board

ABSTRACT

A method for manufacturing a flexible circuit board capable of transmitting high frequency signals with reduced attenuation includes providing an inner wiring board including a first conductive wiring layer and a first substrate layer, the first conductive wiring layer including a signal line and two ground lines on both sides of the signal line, the first substrate layer covering a side of the first conductive wiring layer and defining first through holes which expose the signal line; providing two copper clad laminates including a second substrate layer and a copper foil, the second substrate layer having second through hole aligned with the first through holes; laminating the two copper clad laminates onto two sides of the inner wiring board via two adhesive layers, each adhesive layer defining third through holes aligned with the first and second through holes; and forming a second conductive wiring layer from the copper foil.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of patent application Ser. No.16/916,067 filed on Jun. 29, 2020, which is based on and claims priorityto China Patent Application No. 202010464183.8 filed on May 27, 2020,the contents of which are incorporated by reference herein.

FIELD

The subject matter herein generally relates to circuit boards, inparticular to a method for manufacturing a flexible circuit board.

BACKGROUND

In high-frequency electronic signal transmissions, attenuation of thetransmission signal is mainly a result of dielectric loss. Dielectricloss is positively correlated with dielectric loss factor and dielectricconstant. In order to reduce the transmission loss, a liquid crystalpolymer with a low dielectric constant can be used as the substratelayer covering the signal line. However, such material still has arelatively high dielectric loss.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of the present technology will now be described, by wayof embodiment, with reference to the attached figures.

FIG. 1 is a cross-sectional view of an embodiment of a flexible circuitboard.

FIG. 2 is a top view of a part of a first substrate layer of theflexible circuit board of FIG. 1 .

FIG. 3 is a top view of a part of a first substrate layer in anotherembodiment.

FIG. 4 is a cross-sectional view of an inner wiring board in oneembodiment.

FIG. 5 is a cross-sectional view of a copper clad laminate in oneembodiment.

FIG. 6 is a cross-sectional view showing the copper clad laminate ofFIG. 5 pressed to each side of the inner wiring board of FIG. 4 .

FIG. 7 is a cross-sectional view showing a first conductive hole and asecond conductive hole formed on the structure of FIG. 6 .

DETAILED DESCRIPTION

Implementations of the disclosure will now be described, by way ofembodiments only, with reference to the drawings. The disclosure isillustrative only, and changes may be made in the detail within theprinciples of the present disclosure. It will, therefore, be appreciatedthat the embodiments may be modified within the scope of the claims.

Unless otherwise defined, all technical terms used herein have the samemeaning as commonly understood by one of ordinary skill in the art. Thetechnical terms used herein are to provide a thorough understanding ofthe embodiments described herein, but are not to be considered aslimiting the scope of the embodiments.

FIG. 1 illustrates a flexible circuit board 100 according to oneembodiment. The flexible circuit board 100 includes an inner wiringboard 10, and two outer wiring boards 30 disposed on opposite twosurfaces of the inner wiring board 10.

The inner wiring board 10 includes a first substrate layer 11 and afirst conductive wiring layer 13 disposed on a surface of the firstsubstrate layer 11.

The first conductive wiring layer 13 is made of copper, and includes asignal line 131 and two ground lines 132. The two ground lines 132 arearranged at intervals on both sides of the signal line 131. In theembodiment, the first conductive wiring layer 13 includes two signallines 131 and two ground lines 132 arranged at intervals on both sidesof the two signal lines 131. The first conductive wiring layer 13 has aplurality of slots 134 passing through the first conductive wiring layer13. One slot 134 isolates the signal lines 131 from each other, andother slots 134 isolate the ground lines 132 from the adjacent signalline 131.

The first substrate layer 11 covers a surface of the first conductivewiring layer 13. The first substrate layer 11 defines a plurality offirst through holes 112 corresponding to the signal lines 131, and thesignal lines 131 are exposed from the first through holes 112. Each ofthe first through holes 112 passes through a first surface and a secondsurface of the first substrate layer 11, where the first surface abutsthe first conductive wiring layer 13, and the second surface is arrangedaway from the first conductive wiring layer 13. Referring to FIG. 2 ,the first through holes 112 are arranged in a matrix.

In an alternative embodiment, referring to FIG. 3 , each of the firstthrough holes 112 extends to two adjacent lateral surfaces of the firstsubstrate layer 11. The first through holes 112 may be parallel to orintersect each other. In an alternative embodiment, the first throughholes 112 may be divided into two groups, the first through holes 112 ineach of the two groups are arranged to be parallel to each other, butthe two groups intersect each other to form a grid.

The first substrate layer 11 is made of materials for high frequencytransmissions, such as liquid crystal polymer, polytetrafluoroethylene,poly-ether-ether-ketone, or polyphenylene oxide. The first substratelayer 11 may also be made of other flexible insulation materials, suchas polyimide, polyethylene terephthalate, polyethylene naphthalate, orpolyethylene.

The two outer wiring boards 30 are arranged to outer sides of the firstsubstrate layer 11 and the first conductive wiring layer 13. Each of thetwo outer wiring boards 30 includes a second substrate layer 31 and asecond conductive wiring layer 33. The second substrate layer 31 is incontact with the inner wiring board 10, and the second conductive wiringlayer 33 covers a surface of the second substrate layer 31 away from theinner wiring board 10.

The second substrate layer 31 has a plurality of second through holes312, the second through holes 312 communicate one-to-one with the firstthrough holes 112. In the embodiment, each of the second through holes312 is aligned with one of the first through holes 112. The secondthrough holes 312 may be arranged in a matrix, be parallel to eachother, or may intersect each other.

The second substrate layer 31 may be made of materials for highfrequency transmissions, such as liquid crystal polymer,polytetrafluoroethylene, poly-ether-ether-ketone, or polyphenyleneoxide. The second substrate layer 31 may also be made of other flexibleinsulation materials, such as polyimide, polyethylene terephthalate,polyethylene naphthalate, or polyethylene.

The second substrate layer 31 and the first substrate layer 11 may bemade of the same or different materials. In the embodiment, the secondsubstrate layer 31 and the first substrate layer 11 are both made ofliquid crystal polymer.

The second conductive wiring layer 33 is made of copper, and includes ashielding layer 331 and two circuit layers 332. The two circuit layers332 are arranged at intervals on both sides of the shielding layer 331.The shielding layer 331 is arranged to correspond to the signal line131, and the two circuit layers 332 are arranged to correspond to thetwo ground lines 132. The shielding layer 331 is a copper plating layeror a copper foil. The circuit layer 332 is provided with circuitpatterns.

In an alternative embodiment, the flexible circuit board 100 furtherincludes two adhesive layers 40. The two adhesive layers 40 attach thetwo outer wiring boards 30 to the inner wiring board 10. One adhesivelayer 40 is sandwiched between the first substrate layer 11 and thesecond substrate layer 31 of one outer wiring board 30, and the otheradhesive layer 40 is sandwiched between the first conductive wiringlayer 13 and the second substrate layer 31 of the other outer wiringboard 30. The adhesive layer 40 on the first conductive wiring layer 13infills the slots 134. Each of the two adhesive layers 40 has aplurality of third through holes 42 respectively connecting the firstthrough holes 112 and the second through holes 312. In the embodiment,each of the third through holes 42 is aligned with one of the firstthrough holes 112 and one of the second through holes 312. The thirdthrough holes 42 may be arranged in a matrix, be parallel to each other,or may intersect each other.

The flexible circuit board 100 further includes a plurality of firstconductive holes 50 and a plurality of second conductive holes 60. Thefirst conductive holes 50 are located on both sides of the signal line131, and electrically connect the shielding layers 331 of the outerwiring boards 30, to prevent external electromagnetic interference inthe signal line 131. Specifically, each of the first conductive holes 50passes through one of the slots 134, the first substrate layer 11 of theinner wiring board 10, the second substrate layers 31 of the outerwiring boards 30, and the adhesive layers 40, and electrically connectsthe shielding layers 331 of the outer wiring boards 30. The firstconductive holes 50 may be formed of vias passing through the innerwiring board 10, the adhesive layers 40, and the outer wiring boards 30,the vias then being infilled or electroplated with conductive materials.

The second conductive holes 60 electrically connect the ground lines 132and the circuit layers 332 of the outer wiring boards 30. Specifically,one of the second conductive holes 60 passes through the circuit layer332 and the second substrate layer 31 of one outer wiring board 30, oneadhesive layer 40, and the first substrate layer 11 of the inner wiringboard 10, and electrically connects the circuit layer 332 of the oneouter wiring board 30 and one of the ground lines 132. The other one ofthe second conductive holes 60 passes through the other adhesive layer40 and the circuit layer 332 and the second substrate layer 31 of theother outer wiring board 30, and electrically connects the circuit layer332 of the other outer wiring board 30 to the other one of the groundlines 132. The second conductive holes 60 may be formed as blind holeson the inner wiring board 10, the adhesive layers 40, and the outerwiring boards 30, the blind holes then being infilled or electroplatedwith conductive materials.

The flexible circuit board 100 further includes two protective layers70. The two protective layers 70 are arranged on two outer sides of thetwo outer wiring boards 30, and configured to protect the secondconductive wiring layers 33. In the embodiment, the protective layers 70are covering layers. In other embodiment, the protective layers 70 maybe solder resist layers. The two protective layers 70 may be attached tothe outer sides of the two outer wiring boards 30 by two adhesive layers40.

One embodiment of a method for manufacturing a flexible circuit boardincludes the following steps of:

S1, providing an inner wiring board including a first substrate layerand a first conductive wiring layer on a surface of the first substratelayer, the first conductive wiring layer including a signal line and twoground lines arranged at intervals on both sides of the signal line, thefirst substrate layer having a plurality of first through holescorresponding to the signal line;

S2, providing two copper clad laminates, each of the two copper cladlaminates including a second substrate layer and a copper foil on asurface of the second substrate layer, the second substrate layer havinga plurality of second through holes;

S3, pressing the two copper clad laminates on two sides of the innerwiring board, the second substrate layer abutting the inner wiringboard, and the second through holes communicating one-to-one with thefirst through holes;

S4, forming a second conductive wiring layer from the copper foil toobtain the flexible circuit board.

Referring to FIG. 4 , in step S1, an inner wiring board 10 is provided.The inner wiring board 10 includes a first substrate layer 11 and afirst conductive wiring layer 13 disposed on a surface of the firstsubstrate layer 11. The first conductive wiring layer 13 includes asignal line 131 and two ground lines 132 arranged at intervals on bothsides of the signal line 131. The first substrate layer 11 defines aplurality of first through holes 112 corresponding to the signal lines131, the signal lines 131 are exposed from the first through holes 112.

The first conductive wiring layer 13 is made of copper. In theembodiment, the first conductive wiring layer 13 includes two signallines 131 and two ground lines 132 arranged at intervals on both sidesof the two signal lines 131. The first conductive wiring layer 13 has aplurality of slots 134 passing through the first conductive wiring layer13. One slot 134 isolates the signal lines 131 from each other, andother slots 134 isolate the ground lines 132 from the adjacent signalline 131.

The first substrate layer 11 covers a surface of the first conductivewiring layer 13. Each of the first through holes 112 passes through afirst surface and a second surface of the first substrate layer 11,where the first surface abuts the first conductive wiring layer 13, andthe second surface is arranged away from the first conductive wiringlayer 13.

The first through holes 112 may be arranged in a matrix, be parallel toeach other, or may intersect each other. The first through holes 112 maybe formed by laser cutting or punching.

The first substrate layer 11 may be made of materials for high frequencytransmissions, such as liquid crystal polymer, polytetrafluoroethylene,poly-ether-ether-ketone, or polyphenylene oxide. The first substratelayer 11 may also be made of other flexible insulation materials, suchas polyimide, polyethylene terephthalate, polyethylene naphthalate, orpolyethylene.

Referring to FIG. 5 , in step S2, two copper clad laminates 80 areprovided. Each of the two copper clad laminates 80 includes a secondsubstrate layer 31 and a copper foil 81 on a side of the secondsubstrate layer 31. The second substrate layer 31 has a plurality ofsecond through holes 312.

The second through holes 312 may be arranged in a matrix, be parallel toeach other, or may intersect each other. The second through holes 312may be formed by laser cutting or punching.

The second substrate layer 31 may be made of materials for highfrequency transmissions, such as liquid crystal polymer,polytetrafluoroethylene, poly-ether-ether-ketone, or polyphenyleneoxide. The second substrate layer 31 may also be made of other flexibleinsulation materials, such as polyimide, polyethylene terephthalate,polyethylene naphthalate, or polyethylene.

The second substrate layer 31 and the first substrate layer 11 may bemade of the same or different materials. In the embodiment, the secondsubstrate layer 31 and the first substrate layer 11 are both made ofliquid crystal polymer.

Referring to FIG. 6 , in step S3, the two copper clad laminates 80 arepressed on opposite two sides of the inner wiring board 10, the secondsubstrate layer 31 abuts the inner wiring board 10, and the secondthrough holes 312 communicate with the first through holes 112. In theembodiment, each of the second through holes 312 is aligned with one ofthe first through holes 112.

In an alternative embodiment, the two copper clad laminates 80 areadhered to two sides of the inner wiring board 10 by two adhesive layers40. One adhesive layer 40 is sandwiched between the first substratelayer 11 and the second substrate layer 31 of one copper clad laminate80, and another adhesive layer 40 is sandwiched between the firstconductive wiring layer 13 and the second substrate layer 31 of theother copper clad laminate 80. The adhesive layer 40 on the firstconductive wiring layer 13 infills the slots 134. Each of the twoadhesive layers 40 has a plurality of third through holes 42 connectingthe first through holes 112 and the second through holes 312. In theembodiment, each of the third through holes 42 is aligned with one ofthe first through holes 112 and one of the second through holes 312. Thethird through holes 42 may be arranged in a matrix, be parallel to eachother, or may intersect each other.

Referring to FIG. 7 , after step S3, the method further includes a stepof forming a plurality of first conductive holes 50 and a plurality ofsecond conductive holes 60. The first conductive holes 50 are on bothsides of the signal line 131 and electrically connect the copper foils81 of the copper clad laminates 80, the second conductive holes 60electrically connecting one copper foil 81 and one ground line 132.

Specifically, each of the first conductive holes 50 passes through oneof the slots 134, the first substrate layer 11 of the inner wiring board10, the copper clad laminates 80, and the adhesive layers 40, andelectrically connects the copper foils 81. The first conductive holes 50may be formed of vias passing through the inner wiring board 10, theadhesive layers 40, and the copper clad laminates 80, the vias thenbeing infilled or electroplated with conductive materials.

Specifically, one of the second conductive holes 60 passes through onecopper clad laminate 80, one adhesive layer 40, and the first substratelayer 11 of the inner wiring board 10, and electrically connects the onecopper clad laminate 80 and one of the ground lines 132. The other oneof the second conductive holes 60 passes through the other adhesivelayer 40 and the other copper clad laminate 80, and electricallyconnects the other copper clad laminate 80 and the other one of theground lines 132. The second conductive holes 60 may be formed fromblind holes on the inner wiring board 10, the adhesive layers 40, andthe copper clad laminates 80, the blind holes then being infilled orelectroplated with conductive materials.

Referring to FIGS. 1 and 7 , in step S4, two second conductive wiringlayers 33 are formed on the two copper foils 81 to obtain a flexiblecircuit board 100.

The second conductive wiring layer 33 includes a shielding layer 331 andtwo circuit layers 332. The two circuit layers 332 are arranged atintervals on both sides of the shielding layer 331. The shielding layer331 is arranged to correspond to the signal line 131 to prevent externalelectromagnetic interference affecting the signal line 131. The circuitlayers 332 are arranged to correspond to the ground lines 132, andprovided with circuit patterns.

Each of the two second conductive wiring layers 33 and one of the secondsubstrate layers 31 constitute an outer wiring board 30.

After forming the two second conductive wiring layers 33, two protectivelayers 70 are formed on outer sides of the two second conductive wiringlayers 33. The protective layers 70 are configured to protect the twosecond conductive wiring layers 33. In the embodiment, the protectivelayers 70 are covering layers. In other embodiment, the protectivelayers 70 may be solder resist layers. Two adhesive layers 40 attach thetwo protective layers 70 to outer sides of the two outer wiring boards30.

In the flexible circuit board 100, the first substrate layer 11 of theinner wiring board 10 and the second substrate layers 31 of the outerwiring boards 30 all define through holes corresponding to the signalline 131. The signal line 131 is thus at least partially surrounded byair which has a very low dielectric constant, and attenuation of thesignal line 131 during transmission is thereby reduced.

While the present disclosure has been described with reference toparticular embodiments, the description is illustrative of thedisclosure and is not to be construed as limiting the disclosure.Therefore, those of ordinary skill in the art can make variousmodifications to the embodiments without departing from the scope of thedisclosure as defined by the appended claims.

What is claimed is:
 1. A method for manufacturing a flexible circuitboard comprising: providing an inner wiring board comprising a firstsubstrate layer and a first conductive wiring layer on a side of thefirst substrate layer, the first conductive wiring layer comprising asignal line and two ground lines arranged at intervals on both sides ofthe signal line, the first substrate layer having a plurality of firstthrough holes corresponding to the signal line; providing two copperclad laminates, each of the two copper clad laminates comprising asecond substrate layer and a copper foil on a side of the secondsubstrate layer, the second substrate layer having a plurality of secondthrough holes aligned one-to-one with the plurality of first throughholes; laminating the two copper clad laminates onto two sides of theinner wiring board via two adhesive layers, wherein the two adhesivelayers attaches the two copper clad laminates to the inner wiring board,one adhesive layer is sandwiched between the first substrate layer andthe second substrate layer of one outer wiring board, the other adhesivelayer is sandwiched between the first conductive wiring layer and thesecond substrate layer of the other outer wiring board, each of the twoadhesive layers defines a plurality of third through holes each alignedwith a corresponding one of the plurality of first through holes and acorresponding one of the plurality of second through holes; and forminga second conductive wiring layer from the copper foil to obtain twoouter wiring boards, thus to obtain the flexible circuit board.
 2. Themethod of claim 1, further comprising forming a plurality of firstconductive holes and a plurality of second conductive holes, wherein thefirst conductive wiring layer defines a plurality of slots isolating thesignal line from the two ground lines, each of the plurality of firstconductive holes passes through one of the plurality of slots and iselectrically connected to the second conductive wiring layer of each ofthe two outer wiring boards, and each of the plurality of secondconductive holes electrically connects one of the two ground lines andthe second conductive wiring layer.
 3. The method of claim 2, whereinthe second conductive wiring layer comprises a shielding layer and twocircuit layers arranged at intervals on both sides of the shieldinglayer, the shielding layer is arranged to correspond to the signal line,each of the plurality of first conductive holes is electricallyconnected to the shielding layer of the second conductive wiring layerof each of the two outer wiring boards, and each of the plurality ofsecond conductive holes is electrically connected to a correspondingcircuit layer.
 4. The method of claim 3, wherein the shielding layer isa copper plating layer or a copper foil.
 5. The method of claim 2,wherein forming a plurality of first conductive holes comprises: forminga plurality of vias passing through the inner wiring board, the twoadhesive layers, and the two outer wiring boards, infilling orelectroplating the plurality of vias with conductive materials.
 6. Themethod of claim 1, further comprising: forming a protective layer on anouter side of the second conductive wiring layer.
 7. The method of claim1, wherein each of the plurality of first through holes passes through afirst surface of the first substrate layer adjacent to the firstconductive wiring layer and a second surface of the first substratelayer away from the first conductive wiring layer, the plurality offirst through holes are arranged in a matrix.
 8. The method of claim 1,wherein each of the plurality of first through holes extends to twoadjacent lateral surfaces of the first substrate layer, the plurality offirst through holes are parallel to or intersect each other.
 9. Themethod of claim 1, wherein a material of the first substrate layercomprises one or more of liquid crystal polymer,polytetrafluoroethylene, poly-ether-ether-ketone, and polyphenyleneoxide.
 10. The method of claim 1, wherein a material of the secondsubstrate layer comprises one or more of liquid crystal polymer,polytetrafluoroethylene, poly-ether-ether-ketone, and polyphenyleneoxide.